1. Field of the Invention
The present invention relates to projection optical systems used for flat panel display manufacture, and more particularly, to 2× and larger magnification compact projection optical systems for manufacturing of large flat panel displays (FPDs).
2. Related Art
The manufacture of a liquid crystal display, or a flat panel display (FPD) involves a manufacturing process that is similar to that used in semiconductor wafer manufacturing. A substrate is coated with a photo resist. An exposure system is used to project an image of a reticle onto the substrate, so as to expose the photo resist, and create a pattern of circuits on the substrate. When the exposure is finished, and the substrate is packaged, a flat panel display is formed.
Although FPDs have been in production since the late 1980s, the current size requirement are for FPDs of up to 42 inches diagonal, with 54 and 60 inches diagonal under development. (Note that screen size in the United States is usually specified in inches, while optical designs are normally done in millimeters.) This places severe requirements on the optics used in the projection optical system. Specifically, many existing optical designs, if scaled up to 42 inch FPD manufacturing size, become unreasonably large.
Optical designs, operating at 1× or at some other magnification, compatible with producing 42-inch full field scanners, require very large lenses and/or mirrors. To print 42-inch FPDs, the lithography tool must have a slit height of about 525 mm. The requirement that the optical design form be telecentric results in at least one element in the optical design being larger than 525 mm in diameter, with the exact diameter dependent on the optical systems numerical aperture and back working distance.
Reflective optical design forms operating at 1× magnification are successfully being used in the microlithography industry. FIG. 1 is an example of such a conventional system. While the design is capable of meeting the optical requirements for printing FPDs, this design form requires a concave mirror from about 1.2 m in diameter for the 42 inch display to a 1.7 m diameter for a 60 inch display. While fabricating mirrors of this size is possible, they may not be cost effective in a FPD lithography tool.
As shown in FIG. 1, a conventional design includes two spherical mirrors, a primary concave mirror 101, and a secondary convex mirror 102. Note that, as shown in FIG. 1, the primary mirror is used as a reflector twice. A reticle 103 is positioned off axis. The image of the reticle 103 is projected onto a substrate 104.
The mask making infrastructure to manufacture 1× reticles to work with the system in FIG. 1 for 42 to 60 inch FPDs is very limited. Adding the capacity to manufacture masks in the quantities needed will be very expensive and time consuming to develop. Adding the mask infrastructure development cost to the FPD may result in a FPD product cost that is unacceptably high.
Another method for manufacturing 42 inch and larger FPDs that gets around the need for the extremely large optics in FIG. 1 involves stitching together images of small sections of a 1× mask for creating the full FPD size required. A small, single optical system can be used to create the images of the mask sections one at a time. This is a very time consuming process to create the full FPD.
Another approach uses multiple small optical systems that simultaneously image the 1× masks onto the FPD substrate. A multiple channel system like this is described in published U.S. patent application No. 2003/0137644. This multiple column optical system has more error sources than a single optical system designed and sized to image the full reticle.
Printing FPDs using a stitching based imaging process results in low yields, and therefore high costs, during the manufacturing process. This is due to the fact that the type of imaging errors that occur during the stitching process are easily discernible when viewing large FPDs with a naked eye. To minimize stitching errors, sub-micron accuracy alignment of optical systems is required, which is not normally used in FPD manufacturing, due to cost considerations. As the FPD size increases the stitching related errors also increase resulting in ever more complicated FPD exposure tools.
It is generally thought that a cost effective stitching imaging technique for fabricating 42-inch FPDs will not work. FPD manufactures generally prefer full field scanning systems to any optical design configuration that employs stitching.
Other FPD lithography tools use a 1× magnification full field scan with a two mirror design first described in U.S. Pat. No. 3,748,015.
In the two-mirror approach of FIG. 1, a slot is imaged. This slot is the width of the 32 inch diagonal screen by a few millimeters. The slot is then scanned along the long side of the screen. For the 32-inch FPD a significant improvement in optical manufacturing capability and infrastructure was needed in order to be able to fabricate an approximately 800 mm diameter mirror required for the scanner optical system design.
A 42 inch screen is about 525 millimeters by 930 millimeters. This size reticle can only be produced in very limited volumes making it impractical to use where large quantities of 42 inch display need to be manufactured.
For FPDs with 54- and 60-inch diagonal dimensions the reticle sizes needed for a 1× lithography tool are about 1194×574 mm and about 1328×747 mm. These sizes are beyond the current capability of reticle manufacturers. Significant research and development activities are required to develop mask writing tools in these sizes. Being able to use currently available reticle sizes to fabricate 54- and 60-inch FPDs offers many advantages.
Accordingly, it is desirable to have a projection optical system and an exposure system that can be used in the manufacture of large-scale FPDs, such as 42-inch, 54-inch, and 60-inch FPDs, currently contemplated by FPD manufacturers. It is also desirable to have a projection optical system that can use mask sizes compatible with the current masking manufacturing commercial infrastructure, which is capable of producing masks with diagonal dimensions of up to 32 inches.